The ink-jet printing (recording) system is a system wherein ink droplets are jetted onto a recording sheet such as paper utilizing various working principles to perform recording of images or characters on the sheet. The ink-jet recording system is characterized in that high-speed, low-noise and multicolor recording is feasible, flexibility of recording pattern is great, and developing and fixing processes are unnecessary. Therefore, the ink-jet recording system has rapidly spread as a system for recording various figures (e.g., Chinese characters) and color images, and is applied to various uses.
Further, rapid progress of ink-jet printers used for the ink-jet recording system has been made in these several years, and printers of high resolution, e.g., 720 to 1,440 dpi, have been developed.
With variation of use applications of the ink-jet printing system, recording media used for the system have been desired to have higher printing qualities. For example, the following qualities are desired.
(1) The ink is absorbed quickly, the printed dots are fine, and the color tone is bright and vivid.
(2) The ink is absorbed quickly, and there is no running or bleeding of ink even when the printed dots are overlapped.
(3) After printing, the printed surface of the recording medium has excellent water resistance.
The term "water resistance" used herein means such a stable image-receiving property that no bleeding or running of ink takes place when the recording medium is exposed to moisture after completion of printing.
The recording materials used for the ink-jet printing system are broadly divided into two types, namely, plane paper type, such as wood free paper and bond paper, and coated type wherein an ink-receiving layer is provided on a surface of a support (substrate) made of paper (e.g., wood free paper), synthetic paper, synthetic resin film or the like.
The ink used for the ink-jet printing system generally has, as a counter ion, a cation which is substituted due to change of pH to vary the ink to water-soluble one. On a common resin substrate, however, the ink has poor affinity for the resin support and poor retention of a coloring matter transferred, resulting in various problems. For example, the substrate sheds ink, a defaced image is formed because of excessive ink, or a non-printed area is produced because of insufficient printing.
To cope with these problems, a trial of providing an ink-receiving layer comprising a water-soluble polymer, such as, polyvinyl alcohol, starch, gelatin, cellulose derivative (e.g., hydroxyethyl cellulose, methyl cellulose), poly(meth)acrylic acid or a salt thereof, on a surface of the substrate has been proposed to improve the ink absorption of the substrate.
In Japanese Patent Laid-Open Publication No. 135785/1986 (Japanese Patent Publication No. 15747/1992), a recording medium containing a hydrotalcite compound represented by Mg.sub.6 Al.sub.2 (OH).sub.16 CO.sub.3.4H.sub.2 O or Mg.sub.4-5 Al.sub.2 (OH).sub.16 CO.sub.3.mH.sub.2 O (m is 3 to 3.5) is proposed. The hydrotalcite compound is known as a layered compound, and the ink absorption of the substrate is improved by allowing the layered compound to occlude an anionic coloring matter between layers of the compound.
The hydrotalcite has a polyvalent anion such as a divalent anion (CO.sub.3.sup.2-) or a higher-valent anion, and the polyvalent anion is firmly fixed between layers. Therefore, such hydrotalcite has poor capability of occluding the anionic coloring matter and cause bleeding in the ink-jet printing process.
Further, as ink for the color ink-jet printing system, a fine particle pigment came to be used. In some cases, however, the pigment ink is not uniformly absorbed because of rough surface of the ink-receiving layer or variability of chemical properties of the ink-receiving layer surface. In order to perform uniform printing with high chromaticness, therefore, it is desired that the hydrotalcite particles are of extremely small sizes and are uniformly dispersed in the binder.
Under such circumstances as described above, the present inventors have earnestly studied and have found that a layered compound, such as a hydrotalcite or a hydrotalcite-like compound having a similar structure to that of hydrotalcite (both being generically referred to as "hydrotalcite type compound" hereinafter), in which exchangeable anions are present between layers and at least a part of said exchangeable anions (interlayer anions) are monovalent anions, shows excellent ink absorption and a function of firmly fixing coloring matters. The present inventors have also found that, utilizing the above properties of the hydrotalcite type compound, the clearness and sharpness of an image can be improved by uniformly coating a substrate with the hydrotalcite type compound in the form of a fine powder. Based on the finding, the present invention has been accomplished.